1. Field of the Invention
The present invention relates to a sprocket wheel and a photo film advancing device. More particularly, the present invention relates to a sprocket wheel for use with a photo film in which each of frames is associated with two perforations regularly, and a photo film advancing device for advancing the photo film.
2. Description Related to the Prior Art
A lens-fitted photo film unit, as known in the art, is pre-loaded with unexposed photo film and incorporates a photo film advancing device. The photo film advancing device has a construction in which a sprocket wheel is rotated by movement of photo film, and adapted to charging a shutter. Such a photo film advancing device is also utilized in a simplified camera of a low-cost type. In view of the photo film of a 135 type having eight perforations per frame, the sprocket wheel in the photo film advancing device has eight teeth. Upon advancement of the photo film as much as eight perforations, the sprocket wheel makes one rotation. Thereupon, a manually operable winding wheel is stopped from rotating, to stop the photo film. In the course of the one frame feeding, the shutter is charged. In the photo film advancing device, the advancement of the photo film is mechanically detected by use of the perforations. Only a simple structure can detect the one frame feeding exactly. At least one of the teeth of the sprocket wheel is engaged with a train of the perforations, so that the photo film advancing device can receive relatively high drive force from the photo film. It is possible with reliability to charge the shutter while displacing a spring.
U.S. Pat. No. 5,471,265 (corresponding to JP-A (Japanese Patent Laid-open Publication No.) 5-19368) discloses a photo film in which each frame is associated with two perforations. One of the two is associated with an edge of the frame nearer to the leader. The other of the two is associated with an edge of the frame nearer to the trailer. Each frame is created between positions of the two perforations. The perforations are arranged at two intervals which are alternately repeated. The longer of the two intervals is related to a length of each frame. The shorter of the two intervals is related to an interval between two adjacent frames.
To detect the one frame feeding of the photo film of this type, it is conceivable to monitor passage of the perforations through a photo sensor to stop of the photo film. One of every two of the perforations may be detected photoelectrically. Optionally each pair of perforations may be detected photoelectrically. However, such techniques are unsuitable for a compact camera of a low-cost type, because complicated circuits for amplifying and rectifying a photoelectric signal from the photo sensor would be required as well as the photo sensor.
It is also conceivable to use a detector claw used in a 110 camera of a simple type. The detector claw may be biased by a spring, and mechanically shifted to move into each perforation. When the detector claw enters a perforation at an edge of each frame nearer to the trailer, one frame feeding of the photo film can be detected. This can be detected also upon movement as far as a regular length after entry of the detector claw into a perforation. However, a shortcoming lies in that a mechanism is inevitably required for removing the detector claw from the perforations. Such a removing mechanism, with its excessively complicated linkages, is likely to operate erroneously. Another disadvantage lies in efficient use of interior space, because the claw must be moved perpendicularly to the movement of the photo film.